adsorption of lanthanides iii from aqueous solutions by fullerene black modified with di 2 ethylhexyl phosphoric acid

The effect of HNO3 concentration in the aqueous phase and that of D2EHPA concentration in the sorbent phase on the adsorption of microquantities of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho

* E-mail: karan@iptm.ru

1 Introduction

It is now widely accepted that the use of adsorbents

in metal recovery offers many advantages over

the use of liquid-liquid extraction The most important of

these advantages are the simplicity of equipment and

operation, and the possibility of using a solid adsorbent

for many extraction cycles without losses in the metal

extraction capacity Unfortunately, the preparation of

ion exchangers containing chelating groups connected

to a solid matrix by chemical bonds is usually very

complicated, expensive, and time consuming Therefore,

the concept of using solvent impregnated sorbents was

put forward and developed in [1-3] This is a very simple

and in many cases the only way to prepare ion exchange

sorbents containing reactive groups with special

properties, which cannot be immobilized by chemical

bonding The method includes the incorporation of

an extractant by a physical impregnation technique into

a solid matrix The use macroporous polymeric sorbents

[1-3] and hydrophobized silica gels [4] impregnated with

extractants of various nature for the extraction of metal

ions from aqueous solutions was earlier described One

of the requirements imposed on the solid matrix is that

it should possess a fairly large capacity with respect to

an extractant, which, in turn, determines the capacity

of an adsorbent with respect to the metal ion to be extracted This requirement is usually fulfilled when materials with a high specific surface area are used as solid matrixes In this respect, the fullerene black (FB)

is a good candidate for the preparation of impregnated sorbents, because this material has a comparatively high specific surface area [5]

FB, a new member in the carbon family, is an amorphous product of electric arc graphite vaporization after extraction of fullerenes Unlike graphite and glassy carbon, FB is readily oxidized by dioxygen, brominated and hydrogenolyzed [5] FB is a promising catalyst for dehydrocyclization of alkanes [5,6] and activation of methane [7] The high-temperature behavior of FB [8 and ESR study of the product [9] were described Recently, FB has been found to be an efficient adsorbent for organic solvents (crude petroleum, oils, and

Adsorption of lanthanides(III) from aqueous solutions by fullerene black modified with

di(2-ethylhexyl)phosphoric acid

Received 23 June 2008; Accepted 20 October 2008

Abstract: Fullerene black (FB) - a product of electric arc graphite vaporization after extraction of fullerenes - was modified with the

di(2-ethylhexyl)phosphoric acid (D2EHPA) The distribution of D2EHPA between FB and aqueous HNO3 solutions has been studied The effect of HNO3 concentration in the aqueous phase and that of D2EHPA concentration in the sorbent phase on the adsorption of microquantities of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y nitrates from HNO3solutionsbyD2EHPA-modifiedFB areconsidered.Thestoichiometryofthesorbedcomplexeshasbeendeterminedbytheslopeanalysismethod.Theefficiencyof lanthanides’ adsorption increases with an increase in the element atomic number A considerable synergistic effect has been observed

upon the addition of the neutral bidentate tetraphenylmethylenediphosphine dioxide ligand to D2EHPA in the sorbent phase

© Versita Warsaw and Springer-Verlag Berlin Heidelberg.

Keywords: Fullerene black • Modification • Adsorption • Di(2-ethylhexyl)phosphoric acid • Tetraphenylmethylenediphosphine dioxide •

Lanthanides.

a Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka 142432, Russia

b Institute of Microelectronics Technology and High Purity Materials,

Russian Academy of Sciences, Chernogolovka 142432, Russia

A N Turanova, V K Karandashevb*

Research Article

chlorobenzene) from aqueous emulsions [5]

Impregnated with

1-phenyl-3-methyl-4-benzoylpyrazol-5-one, FB also showed a high adsorption efficiency

for U(VI), Th(IV), Zr(IV), Sc(III), and lanthanides(III)

recovery from aqueous solutions [10]

The aim of this work was to study the adsorption ability

of FB towards phosphororganic acidic extractant,

di(2-ethylhexyl)phosphoric acid (D2EHPA), and to

estimate the feasibility of D2EHPA-modified FB for the

adsorption of lanthanides(III) from nitric acid solutions

2 Experimental Procedures

FB was prepared as described in [5] The specific

surface area of FB determined by the BET method

was 274 m2 g-1 Analytical grade D2EHPA was purified

according to [11] Tetraphenylmethylenediphosphine

dioxide (TPMDPDO) was synthesized by the known

method [12] and purified by crystallization

The FB-D2EHPA sorbents were prepared according

to the principles of the dry impregnation method [2]

An appropriate amount of FB (2 - 3 g) was placed

in a round-bottomed flask and dichloromethane

containing D2EHPA of different concentrations was

added The mixture was equilibrated for 12 hours

on a rotary evaporator without applying a vacuum

Then dichloromethane was removed by applying

a controlled vacuum and the sorbent was further dried

to constant weight The concentrations of D2EHPA in

the sorbent were varied from 0.1 to 1.5 mmol g-1

The same procedure was followed to prepare FB

impregnated with TPMDPDO and with a mixture of

D2EHPA and TPMDPDO

In order to investigate the retention of D2EHPA on FB

and its distribution between the sorbent phase and

the aqueous phase as a function of HNO3 concentration

in the aqueous phase and that of D2EHPA in the sorbent

phase, batch experiments were carried out at 20 ± 2oC

In these experiments 0.1 g of dry FB-D2EHPA and 10 mL

of the aqueous phase were stirred in stoppered

glass tubes for 2 h The concentration of HNO3 in

the aqueous phase was varied between 0.003 and 1 M

The suspensions were then filtered through membrane

filters and the total concentration of D2EHPA in

the aqueous phase was determined by inductively

coupled atomic emission spectrometry (ICP-AES)

on an ICAP-61 spectrometer (Thermo Jarrell Ash, USA)

The content of D2EHPA in the sorbent phase was evaluated

from the material balance between the initial extractant

concentration in the sorbent phase and that found

in the aqueous phase after equilibration The distribution

ratio (D) was calculated as the ratio of concentrations

in the equilibrium solid and aqueous phases

Aqueous solutions of lanthanide nitrates were prepared

by dissolving the corresponding oxides in high purity nitric acid The distribution of La, Ce, Pr, Nd, Sm, Eu,

Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y in the adsorption systems was studied in model solutions of nitric acid

of variable concentrations at the initial metal concentration (2 ± 0.1) × 10-5 M for each element

The experiments on the adsorption of metal ions were performed in the static mode at 20 ± 2oC A weighed sample (0.1 g) of the sorbent was mixed with an Ln aqueous solution (10 mL) for 1 h; this was time found earlier to be sufficient for the system to reach equilibrium

Preliminary experiments showed that the adsorbtion

of lanthanides(III) onto FB from HNO3 solutions in the absence of D2EHPA is negligible

Isotherms of Eu(III) adsorption were studied by adding 0.1 g of the FB-D2EHPA sorbent to 10 ml solutions with initial concentrations of Eu(III) from 2 × 10-5 to 1 × 10-3 M

at varied D2EHPA concentrations (0.3, 0.5 and 1.0 M)

in the sorbent phase

Lanthanide concentrations in the initial and equilibrium aqueous solutions were determined by inductively coupled plasma mass-spectrometry (ICP-MS) on

a PlasmaQuad (VG Elemental, GB) following the procedure described in [13] The concentration of lanthanides in the sorbent phase was found by the material balance equation The distribution ratios lanthanides (DLn) were calculated as the ratio of concentrations in the equilibrium solid and aqueous phases Duplicate experiments showed the reproducibility of the DLn measurements was generally within 10% HNO3 concentration in the equilibrium aqueous solutions was determined by potentiometric titration with KOH solution and pH was measured on a pH meter (pH-150, Russia) equipped with a combined glass electrode

3 Results and discussion

The distribution of D2EHPA (HA) between the loaded FB and the aqueous solution was investigated for different loading amounts of the extractant in the sorbent phase

at different HNO3 concentrations in the aqueous phase

The distribution ratio of HA (DHA) rises with an increase

of HNO3 concentration in the equilibrium aqueous phase (Fig 1), because HA is largely sorbed in a nondissociated form Far from the saturation concentration of HA in the sorbent, the variation of DHA with H+ ions concentration

in the aqueous phase can be described by the equation

DHA = KHA(1 + Ka[H+]-1)-1 , (1) where KHA is the distribution constant of HA and Ka is the ionization constant of HA (pKa = 1.3 [14])

The interphase distribution of HA at [H+] >> Ka, when

HA in the aqueous phase is nondissociated, can be described by the Langmuir equation

[HA] = KHA[HA]max[HA](1 + KHA[HA])-1 (2) where [HA] and [HA] are the equilibrium HA concentrations in the aqueous and solid phases and [HA]max is the maximum HA concentration in the sorbent for monolayer adsorption From the experimental data presented in Fig 2, through linearizing Eq (2) as 1/DHA versus [HA], we derived [HA]max = 2.15 mmol g-1

and log KHA = 3.88 The value of the distribution constant for D2EHPA on FB is higher than the corresponding value for D2EHPA in hexane (log KHA = 3.48 [14]) This indicates that the interphase equilibrium of D2EHPA

is considerably shifted to the sorbent phase, and it seems that the interaction of D2EHPA with FB acts

to further drive the displacement of D2EHPA molecules from the aqueous solution towards the sorbent phase

It follows from Eq (2) that

DHA = KHA([HA]max - [HA]) (3) that is, HA transfer into the aqueous phase is enhanced when the HA concentration in the sorbent phase increases or, accordingly, as the free surface area of the FB matrix becomes smaller

The adsorption of lanthanides(III) by the FB-D2EHPA sorbent decreases with an increase of aqueous HNO3 concentration (Fig 3)

The dependence logDLn = f(log[H+]) is linear with a slope

of -3 Therefore, the adsorption of the metal ion is accompanied by a release of free protons Considering that the resulting Ln(III) complex may be solvated

by nondissociated HA [15], the adsorption of lanthanide ions can be described by the following general expression:

Ln3+ + mHA = LnA3(HA)m-3 + 3H+ (4) where the overboarded formulas refer to the sorbent phase The equilibrium constant of the above reaction is

(5)

KLn=[LnA3(HA)m-3][H+]3[Ln3+]-1[HA]-m=DLn [H+]3[HA]-m

From Eq (5), the following relationship can be obtained logDLn = log KLn + mlog[HA] - 3log[H+] (6) This relationship was used to determine the stoichiometry

of the adsorbed complexes

Figure 1. Influence of HNO3 concentration in the aqueous phase

on the distribution of D2EHPA between the FB-D2EHPA sorbent and the

error bars.

Figure 3. The effect of HNO3 concentration in the

aque-ous phase on the adsorption of Ln(III) by the FB-D2EHPA sorbent

CHA = 1.0 mmol g -1 The sizes of the points represent error bars

Slope: -3.01 ± 0.12 (Y), -3.0 ± 0.12 (La), -2.98 ± 0.13 (Ce),

-2.98 ± 0.13 (Pr), -3.03 ± 0.15 (Nd), -2.97 ± 0.14 (Sm), -2.98 ± 0.15 (Eu),

-3.03 ± 0.16 (Gd), -3.0 ± 0.15 (Tb), -3.02 ± 0.17 (Dy), -2.91 ± 0.19 (Ho),

-2.97 ± 0.16 (Er), -3.03 ± 0.17 (Tm), -2.98 ± 0.15 (Yb), and -2.94 ± 0.18 (Lu)

Figure 2 The distribution of D2EHPA between the FB-D2EHPA

represent error bars.

Figure 5. Adsorption isotherms of Eu(III) adsorbed by the

concentration: 0.002 M)

Figure 6. The adsorption of lanthanides and yttrium from

D2EHPA and TPMDPO (sorbent dosage: 0.1 g per 10 mL; concentration

repre-sent error bars.

The dependence logDLn = f(log[HA]) is linear with

a slope of 6 (Fig 4) Hence, the LnA3(HA)3 species can be assumed to be present in the sorbent phase The data on loading the FB-D2EHPA sorbent by europium(III)suggest the same stoichiometric ratio in the sorbed Eu(III) complex (Fig 5).The DLn value at the given HNO3 concentration grows in going from La to Lu (Fig 3) with an increasing charge density on the Ln3+ ion, in analogy with the trends observed for the solvent extraction system with D2EHPA [15] The difference in DLn values between Lu(III) and La(III) is fairly large (about 4.7 log units), showing the potential usefulness of the FB-D2EHPA sorbent

as a stationary phase in the chromatographic system

We expected that the replacement of HA solvated molecules in the LnA3(HA)3 complex by a neutral ligand (L) with a greater basicity and lipophilicity than those of

HA would raise DLn if fullerene black impregnated with

a mixture of HA and L is used The stability of the resulting complexes would be governed by the acceptor properties of the LnA3 chelates and the donor power of

a neutral ligand, L [16] In fact, introducing TPMDPO into the sorbent causes a nonadditive increase in DLn (Fig 6) The synergistic effect, S = Dmix/(DHA + DL) (where

DHA, DL, and Dmix are the lanthanide distribution ratio for

FB impregnated with HA, TPMDPO and their mixtures), grows in going from La to Lu, reaching S = 170 for Lu The high complexing power of TPMDPO is apparently due to its bidentate coordination in the resulting complexes [17] Earlier, a similar synergistic effect was observed in the solvent extraction of Eu(III) and Am(III) with a mixture of octyl(phenyl)-N,N-diisobu tylcarbamoylmethylphosphine oxide (CMPO) and bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HR) [18]

or di(chlorophenyl)dithiophosphinic acid [19] It was shown that Eu(III) passes into the organic phase as

an Eu(NO3)R2(CMPO)3-x(H2O)x complex [18] Complexes

of similar composition would probably result when Ln ions are adsorbed from nitric acid solutions by the FB sorbent impregnated with a mixture of HA and TPMDPO

4 Conclusions

Fullerene black is a convenient matrix for the impregnated sorbent preparation The results of HNO3 concentration effect on the distribution ratio of D2EHPA show that

an increase of HNO3 concentration in the aqueous phase leads to the minimization of the extractant loss

The efficiency of lanthanide ions adsorption increases when the concentration of D2EHPA in the sorbent phase increases or when the concentration of HNO3 in the aqueous phase decreases The distribution ratio of lanthanides increases with an increase in the atomic number of an element

Figure 4 The effect of D2EHPA concentration in the sorbent phase

on the adsorption of Ln(III) from 0.05 M HNO3 solutions The sizes of the

points represent error bars Slope: 5.95 ± 0.25 (La), 6.01 ± 0.26 (Ce), 6.02

± 0.21 (Nd), 6.04 ± 0.21 (Sm), 6.02 ± 0.23 (Eu), 6.02 ± 0.20 (Tb), 5.97

± 0.0.26 (Dy), 5.96 ± 0.25 (Ho), 5.96 ± 0.24 (Er), 5.96 ± 0.22 (Tm), and

5.98 ± 0.21 (Lu)

A considerable synergistic effect has been observed

upon addition of the neutral bidentate TPMDPO ligand

to D2EHPA in the sorbent phase

Acknowledgments

We are grateful to T.A Orlova and A.E Lezhnev for

the assistance in ICP-MS measurements

References

[1] A Warshawsky, Trans Inst Min Metall 83, 101

(1974)

[2] A Warshawsky, Extraction with solvent impregnated

resins, In: J.A Marinsky, Y Marcus (Eds.), Ion

Exchange and Solvent Extraction (Marcel Dekker

Inc., New York, 1981) Vol 8, 229

[3] J.L Cortina, A Warshawsky, Developments in

solid-liquid extraction by solvent impregnated resins In:

J.A Marinsky and Y Marcus (Eds.), Ion Exchange

and Solvent Extraction (Marcel Dekker Inc., New

York, 1997) Vol 13, 195

[4] Yu.A Zolotov, G.I Tsisin, S.G Dmitrienko,

E.I Morosanova, Sorption preconcentration of

microcomponents from solutions (Nauka, Moscow,

2007)

[5] S.D Kushch, P.V Fursikov, N.S Kuyuko, A.V

Kulikov, V.I Savchenko, Eurasian Chem Tech J

3, 131 (2001)

[6] P.V Fursikov, S.D Kushch, V.E Muradyan,

G.I Davydova, E.I Knerelman, A.P Moravsky, Mol

Mater 13, 319 (2000)

[7] S.D Kushch, V.E Muradyan, P.V Fursikov, E.I Knerelman, V.L Kuznetsov, Yu.V Butenko, Eurasian Chem Tech J 3, 67 (2001)

[8] D Ugarte, Carbon 32, 1245 (1994) [9] A.V Kulikov, S.D Kushch, P.V Fursikov, V.R Bogatyrenko, Appl Magn Reson 22, 539 (2002)

[10] A.N Turanov, V.K Karandashev, S.D Kushch, P.V Fursikov, Russ J Phys Chem 78, 1298 (2004)

[11] E.S Stoyanov, V.A Mikhailov, V.M Popov, Koord Khim 10, 1619 (1984)

[12] E.N Tsvetkov, N.A Bondarenko, I.G Malahova, M.I Kabachnik, Zh Obshch Khim 55, 11 (1985) [13] A.N Turanov, V.K Karandashev, V.E Baulin, Solvent Extr Ion Exch 14, 227 (1996)

[14] V.S Ulyanov, R.A Sviridova, Sov Radiochem 12,

41 (1970) [15] E.O Otu, A.D Westland, Solvent Extr Ion Exch

8, 759 (1990) [16] O.M Petrukhin, Coordination chemistry and extraction of neutral metal complexes, In: V.A Mikhailov (Ed.), Extraction chemistry (Nauka, Novosibirsk, 1984) 112

[17] A.M Rozen, J Radioanal Nucl Chem 143, 337 (1990)

[18] G Ionova, S Ionov, C Rabbe, C Hill, C Madic,

R Guillaumont, J.C Krupa, Solvent Extr Ion Exch

19, 391 (2001) [19] G Ionova, S Ionov, C Rabbe, C Hill, C Madic,

R Guillaumont, G Modolo, J.C Krupa, New J Chem 25, 491 (2001)